Optical compensated bend mode liquid crystal display

ABSTRACT

An optical compensated bend (OCB) mode liquid crystal display (LCD) includes a pixel electrode, a color filter, a common electrode and a liquid crystal layer. The pixel electrode is formed on the first substrate of the OCB mode LCD. The color filter is formed on the second substrate of the OCB mode LCD. The common electrode is formed on the color filter. The liquid crystal layer is sandwiched between the first substrate and the second substrate. A step structure is formed on the second structure, so that the liquid crystal molecules in the liquid crystal layer are twisted into the bend state from the splay state uniformly and quickly.

This application is a Divisional of copending application Ser. No.11/491,143 filed on Jul. 24, 2006, which claims priority to applicationSer. No. 95/103,616 filed in Taiwan, on Jan. 27, 2006. The entirecontents of all of the above applications are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an optical compensated bend (OCB) modeliquid crystal display (LCD), and more specifically to an OCB mode LCDincluding a display panel that is capable of accelerating the transitionof liquid crystal molecules from splay alignment into bend alignment.

(2) Description of the Prior Art

Liquid crystal displays (LCDs) are compact in size, lightweight, inaddition to low power consumption and lower radiation. Due to thesemeritorious features, LCDs are widely used in personal digitalassistants (PDA), notebook computers, digital cameras, video cams,mobile phones, and many other electronic devices. The manufacturersworldwide have devoted themselves to further research and thus improvethe materials, processes and equipments. The display qualities of LCDsare accordingly and largely promoted while the cost goes downday-by-day, which, in turn causes a wider use of LCD.

In order to improve the response speed of the liquid crystal moleculesand to widen the viewing angle of the display panel, researches relatedto the material characteristics of the liquid crystal molecules areconducted. Presently, three methods are proposed, namely: (1) thevertical alignment liquid crystal mode; (2) low-viscosity liquid crystalmolecules; and (3) the optically compensated bend (OCB) mode.

With the vertical alignment mode, the liquid crystal molecules alignwith the alignment film and change their orientation rapidly into thevertical direction when a voltage is applied to the pixel electrode.Research conducted on the low-viscosity liquid crystal materialindicates that the response time is directly related to the viscosity ofthe liquid crystal molecules. Shorter response time can be obtained withlow-viscosity liquid crystal molecules.

In the OCB mode LCD, the liquid crystal molecules near the upper andlower glass substrates are oriented in parallel directions while theliquid crystal molecules therebetween in the liquid crystal layer arenot twisted but are operated in the bent alignment state with respect toa vertical plane. Such type of bent alignment can result in doublerefraction of the light. A biaxial retardation film is generally used tocompensate the axial phase difference so as to overcome the restrictedviewing angle caused by the parallel alignment of the liquid crystalmolecules on the boundaries. In addition, the liquid crystal moleculesin the OCB mode require fast response time of 1-10 ms to switch betweendark and bright state operation when compared to the liquid crystalmolecules of the TN (twisted nematic) mode which require a response timeof about 50 ms.

Note that, though the OCB mode LCD has the aforesaid advantages, therestill exist some disadvantages, such as the requirement of a longerwarming-up time in order to perform the transition of the liquid crystalmolecules in the liquid crystal layer from the splay alignment into thebend alignment. In the presently available OCB mode of LCD, a highvoltage is generally applied to the liquid crystal layer in order toquicken the transition of the liquid crystal molecules of the liquidcrystal layer from the splay alignment into the bend alignment. FIGS. 1Aand 1B respectively show the liquid crystal molecules 10 in the splayalignment state and the bend alignment state. When a predeterminedvoltage is applied on the common electrode 12 and the pixel electrode14, the molecules 10 of FIG. 1A in the splay alignment are changed intothe bent alignment of FIG. 1B after being twisted.

In order to accelerate the transition of the molecules 10 from the splayalignment into the OCB mode, a high voltage is usually applied betweenthe common and pixel electrodes 12, 14 and to the liquid crystal layertherebetween. However, such method cannot uniformly and rapidly transitall the molecules into the required bent mode. Under this condition, thedisplay panel of the LCD is unable to display the images in the normalcondition.

In order to form a larger voltage difference between the commonelectrode 14 and the pixel electrode 12, the design of the driver chipfor the display panel have to be altered, or, the design of the pixelpattern have to be modified. However, these result in extra productioncost. Therefore, the manufacturers are in the trend to produce an LCDhaving a display panel that is capable of accelerating the transition ofthe molecules from the splay alignment into the bent alignment.

SUMMARY OF THE INVENTION

An optical compensated bend (OCB) mode liquid crystal display (LCD) isprovided by the present invention. Liquid crystal molecules in the OCBmode LCD of the present invention are twisted into the bend state fromthe splay state quickly, for displaying an image.

The OCB mode LCD includes a pixel electrode, a color filter, a commonelectrode and a liquid crystal layer. The pixel electrode is formed on afirst substrate of the OCB mode LCD. The color filter is formed on asecond substrate of the OCB mode LCD and corresponds to the pixelelectrode. The common electrode is form on the color filter.

The liquid crystal layer is sandwiched between the pixel electrode andthe common electrode. When an electric field is applied between thepixel electrode and the common electrode, liquid crystal molecules inthe liquid crystal layer are affected by the electric field. The liquidcrystal molecules are twisted into the bend state from the splay state.

What is worth mentioning is that a step structure is disposed on thesecond structure. Through the disposition of the step structure, theliquid crystal molecules are arranged at an angle to an alignmentdirection resulted from an alignment layer in the initial state. As aresult, the liquid crystal molecules remain in a state close totwisting. Also, the step structure changes the distribution of theelectric field in a pixel element, so that the liquid crystal moleculesare affected by the changed electric field to have a pre-inclined angle.

Therefore, when the electric field is applied between the pixelelectrode and the common electrode, the liquid crystal molecules in theliquid crystal layer are twisted into the bend state from the splaystate uniformly and quickly.

These and other objectives of the present invention will no doubt becomeobvious to those of ordinary skill in the art after reading thefollowing detailed description of the preferred embodiment which isillustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be specified with reference to itspreferred embodiment illustrated in the drawings, in which

FIG. 1A and FIG. 1B respectively show the liquid crystal molecules 10 inthe splay alignment state and the bend alignment state.

FIG. 2 is a cross-sectional view of an OCB mode LCD according to thefirst embodiment of the present invention;

FIG. 3A is a top view of the second substrate according to the firstembodiment of the present invention;

FIG. 3B is a cross-sectional view of the second substrate in FIG. 3Ataken along a section line A-A′;

FIG. 3C illustrates a pattern of the step structure disposed on thesecond substrate according to the present invention;

FIG. 4A is a top view of the second substrate according to the secondembodiment of the present invention;

FIG. 4B is a cross-sectional view of the second substrate in FIG. 4Ataken along a section line A-A;

FIG. 5A is a top view of the second substrate of the OCB mode LCDaccording to the third embodiment of the present invention;

FIG. 5B is a cross-sectional view of the second substrate in FIG. 5Ataken along a section line A-A′; and

FIG. 5C illustrates another pattern of the step structure disposed onthe second substrate according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Please refer to FIG. 2. FIG. 2 is a cross-sectional view of an opticalcompensated bend (OCB) mode liquid crystal display (LCD) according tothe first embodiment of the present invention. A liquid crystal display2 is assembled by the first substrate 20 and the second substrate 21. Aliquid crystal layer L is sandwiched between the first substrate 20 andthe second substrate 21. The OCB mode LCD 2 further includes a pixelelectrode 22, a color filter 23 and a common electrode 24.

As shown in FIG. 2, the pixel electrode 22 is formed on the firstsubstrate 20 of the OCB mode LCD 2. In general, other components, suchas scan lines, data lines, thin film transistors and common lines, aredisposed on the first substrate 20. The thin film transistors are turnedon or turned off by the electric signals inputted by the scan lines. Asa result, the voltage signals of the data lines are applied to the pixelelectrode 22.

The color filter 23 is formed on the second substrate 21 of the OCB modeLCD and corresponds to the pixel electrode 22.

The common electrode 24 is formed on the color filter 23. When anelectric field is generated between the pixel electrode 22 and thecommon electrode 24, the liquid crystal molecules in the liquid crystallayer L are affected by the electric field. As a result, the liquidcrystal molecules are twisted into the bend state from the splay state.

What is worth mentioning is that a step structure S is disposed on thesecond substrate 21. Through the disposition of the step structure S,the liquid crystal molecules adjacent to the step structure S in theliquid crystal layer L are affected by the step structure S. As aresult, the initial states of the liquid crystal molecules remain in thesplay state to be twisted into the bend state easily and quickly.Furthermore, through collisions between the liquid crystal molecules,the arrangement of the liquid crystal molecules adjacent to the stepstructure S spreads to the whole liquid crystal layer L. Therefore, eachliquid crystal molecule in the whole liquid crystal layer L remains inthe state to be twisted easily and quickly. When an electric field isformed between the pixel electrode 22 and the common electrode 24, theliquid crystal molecules in the liquid crystal layer L are twisted intothe bend state from the splay state quickly and uniformly.

The step structure S is illustrated further as follow. Please refer toboth FIG. 3A and FIG. 3B. FIG. 3A is a top view of the second substrateaccording to the present invention. FIG. 3B is a cross-sectional view ofthe second substrate in FIG. 3A taken along a section line A-A′.

As shown in FIG. 3B, a black matrix (BM) 25 is formed over the secondsubstrate 21. The color filter 23 is formed over the black matrix 25 sothat the black matrix 25 is sandwiched between the second substrate 21and the color filter 23. The common electrode 24 is formed on the colorfilter 23.

A bump 26 is disposed on the common electrode 24. In a preferredembodiment, the disposition pattern of the bump 26 is shown in FIG. 3A.The bump 26 disposed on the common electrode 24 has a sawtooth patternso that the step structure S (FIG. 3B) is formed on the common electrode24. The bump 26 can be made of an organic material or similar materials.

Through the above structure, the liquid crystal molecules adjacent tothe bump 26 are arranged according to the shape of the bump 26. As aresult, the initial states of the liquid crystal molecules remain in thesplay state to be twisted into the bend state easily and quickly.Meanwhile, this state spreads to the whole liquid crystal layer L sothat the liquid crystal molecules in the whole liquid crystal layerremain in the state to be twisted into the bend state easily andquickly. Therefore, the time to twist the liquid crystal molecules tothe bend state from the splay state is shortened.

What is worth mentioning is that the disposition of the step structure Sformed by the bump 26 is not limited to the sawtooth pattern 26 a. Forexample, the step structure S formed by the bump 26 can be arranged asshown in FIG. 3C. The bump 26 disposed on the common electrode 24 has acontinuous pattern 26 b with several bending sections. Although thecontinuous pattern 26 b with several bending sections in FIG. 3C isillustrated as a right-angle pattern with several bending sections, theembodiments of the invention include an oblique pattern with severalbending sections and an oblique pattern with single bending section.

FIG. 4A is a top view of the second substrate of the OCB mode LCDaccording to the second embodiment of the present invention. FIG. 4B isa cross-sectional view of the second substrate in FIG. 4A taken along asection line A-A.

Generally speaking, the color filter 23 includes several colorresistors, such as a red resistor, a green resistor, and a blueresistor. Light is filtered by the color resistors of different colorsto display a color image. In the present embodiment, the step structureS is formed by overlapping two adjacent color resistors 23 a and 23 b.Specifically speaking, the black matrix 25 is formed over the secondsubstrate 21. The color resistors 23 a and 23 b are formed over theblack matrix 25 respectively to form the step structure S. In thepresent embodiment, the color resistor 23 b overlaps the color resistor23 a. The side of the color resistor 23 b has a sawtooth pattern.

As shown in FIG. 4B, the common electrode 24 is formed over the stepstructure S. In a preferred embodiment, the common electrode 24positioned over the step structure S is conformal to the step structureS. It is to say that the step structure is formed by a portion of theblack matrix and the two overlapping edges of the two adjacent colorresistors, and the edges of the at least two adjacent color resistorsare sawtoothed so as to form a sawtoothed step structure. A shape of thesawtoothed is composed of at least two V linked together.

FIG. 5A is a top view of the second substrate of the OCB mode LCDaccording to the third embodiment of the present invention. FIG. 5B is across-sectional view of the second substrate in FIG. 5A taken along asection line A-A′.

As stated above, the color filter includes several color resistors. Atleast one resistor has an opening 23 c to form the step structure S. Asshown in FIG. 4B, when a color resistor is formed on the black matrix25, the color resistor is cut to form the opening 23 c. As a result, aportion of the black matrix 25 is exposed. Then, the common electrode 24is formed over the color resistor.

In the embodiment illustrated in FIG. 5A, the opening 23 c over thecolor resistor 23 has a sawtooth pattern 23 c 1. In other embodiments,the opening 23 c over the color resistor 23 can be a tooth pattern 23 c2 as shown in FIG. 5C.

Although the openings in FIG. 3A, FIG. 3C, FIG. 4A, FIG. 5A and FIG. 5Chave continuous tooth or sawtooth patterns, the embodiments of theinvention include discontinuous patterns. The shape of the pattern isnot limited to tooth or sawtooth patterns.

Based on the above, the initial states of the liquid crystal moleculesin the OCB mode LCD of the invention remain in a state to be twistedquickly due to the step structure. When an electric field is generatedbetween the common electrode and a pixel electrode, the liquid crystalmolecules are twisted into the bend state from the splay state quicklyand uniformly for displaying an image. Also, the warm-up time of the OCBmode LCD is shortened greatly. Furthermore, according to the presentinvention, the original manufacturing process is not required to bechanged enormously to form the step structure. The step structure iseasily formed and highly practical.

With the example and explanations above, the features and spirits of theinvention are hopefully well described. Those skilled in the art willreadily observe that numerous modifications and alterations of thedevice may be made while retaining the teaching of the invention.Accordingly, the above disclosure should be construed as limited only bythe metes and bounds of the appended claims.

1. An optical compensated bend (OCB) mode liquid crystal display (LCD)comprising: a first substrate; a second substrate; a pixel electrodeformed on the first substrate; a common electrode formed on the secondsubstrate, wherein a bump is disposed on the common electrode; and aliquid crystal layer sandwiched between the first substrate and thesecond substrate.
 2. The OCB mode LCD of claim 1, wherein the bump ismade of an organic material.
 3. The OCB mode LCD of claim 1, wherein thebump disposed on the common electrode has a sawtooth pattern.
 4. The OCBmode LCD of claim 1, wherein the bump disposed on the common electrodehas a continuous tooth pattern.
 5. The OCB mode LCD of claim 1, whereinthe liquid crystal layer includes liquid crystal molecules, and theliquid crystal molecules in the proximity of the bump are aligned withthe shape of the bump.
 6. The OCB mode LCD of claim 1, furthercomprising a color filter formed between the second substrate and thecommon electrode.
 7. The OCB mode LCD of claim 6, further comprising ablack matrix positioned between the second substrate and the colorfilter.